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The iron-nickel based alloys, which are an extension of stainless steel technology, are generally wrought, whereas the cobalt and nickel based alloys are both wrought and cast.1 The iron-nickel based alloys have high strengths below 1200° F and are more easily processed and welded than the nickel based alloys. Cobalt based superalloys have high melting points and high temperature capability at moderate stress levels, excellent hot salt corrosion resistance and better weldability than the nickel based alloys.11 However, they cannot compete with the nickel based alloys at high temperatures and high stress levels.

The most commercially important superalloy, Inconel 718, is listed as an iron-nickel based alloy even though it contains more nickel than iron. This classification fits with the traditional classification for this alloy, although many newer works list it as a nickel based alloy. The significance of Inconel 718 is shown in the Fig. 6.6 material distribution, where it accounts for 34% of the General Electric CF6 engine. Also note that for the cobalt based alloys, there are none listed as being precipitation hardened, because unfortunately, these alloys do not precipitation harden like the nickel and iron-nickel alloys. Also note that the composition of the cast alloys in Table 6.3 is generally more complex than for the wrought alloys.

6.2.1 Nickel Based Superalloys

Nickel based superalloys are the most complex of the superalloys and are used in the hottest parts of aircraft engines, constituting over 50% of the engine weight. They are either solid solution hardened for lower temperature use or precipitation hardened for higher temperature use. The nickel based alloys contain at least 50% nickel and are characterized by the high phase stability of the FCC austenitic (y) matrix. Many nickel based alloys contain 10-20% chromium, up to about 8% aluminum and titanium combined, 5-15% cobalt, and small amounts of boron, zirconium, hafnium and carbon. Other common alloying additions are molybdenum, niobium, tantalum, tungsten and rhenium. Chromium and aluminum are important in providing oxidation resistance by forming the oxides Cr2O3 and Al2O3 respectively.

The most important precipitate in the nickel based alloys is y' in the austenitic y nickel matrix. An example of a y' strengthened alloy is the wrought alloy Waspaloy and the cast alloys René 80 and Inconel 713C. In general, strengthening increases with increasing amounts of y', which is a function of the combined aluminum and titanium content as shown in Figure 6.7. When the volume fraction of y' is less than about 25%, the precipitate particles are spherical, changing to a cubical shape at volume percentages greater than 35%. Most wrought nickel based alloys contain between 20 and 45% y', while cast alloys can contain as much as 60% y'. As the amount of y' increases, the elevated temperature resistance increases.

Alloys containing niobium are strengthened primarily by y''. Alloys can also contain both niobium and titanium and/or aluminum and be strengthened by

Composites Al 4%

Composites Al 4%

Castings 6%
Fig. 6.6. Materials Distribution for GE CF6 Engine11

a combination y'' and y', such as Inconel 706. Some nickel based alloys, such as Hasteloy X and Inconel 625, are predominately hardened by solid solution strengthening and are therefore used at lower temperatures, predominately being specified for corrosive environments. There are also oxide dispersion hardened alloys that are strengthened by a dispersion of inert particles, such as yttria (Y2O3), including MA-754 and MA-6000, where the MA stands for the mechanical alloying process used to prepare metal powders for consolidation.

Table 6.3 Nominal Compositions of Select Cast Superalloys

Alloy Composition

Table 6.3 Nominal Compositions of Select Cast Superalloys

Alloy Composition

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